A fluorinated compound, such as R410 that is an HFC-based refrigerant, has widely been used as conventional refrigerant for a heat pump apparatus of a refrigerator-freezer, an air-conditioning apparatus, or other apparatus. However, such a refrigerant significantly influences global warming. Thus, it is desired to use a refrigerant less influencing global warming from the viewpoint of preventing global warming. Under such a background, there has been proposed use of a refrigerant less influencing global warming, such as R32 that is an HFC-based refrigerant, R1234yf that is an HFO-based refrigerant, and propane or isobutane that is a hydrocarbon-based refrigerant. However, these refrigerants are all flammable refrigerants (or slightly flammable refrigerants) unlike the HFC-based refrigerant that has conventionally been used.
In the heat pump apparatus using these kinds of flammable refrigerants, there is a risk in that refrigerant leaks indoors from a heat exchanger or a pipe constituting a refrigeration cycle to cause an accident, such as fire. In view of the risk, there has been proposed a heat pump apparatus of an indirect heat exchange system, in which the heat pump apparatus is divided into a primary-side refrigeration cycle located on a heat source side and a secondary-side refrigeration cycle located on a load side, thereby preventing the indoor leakage of refrigerant on the load side (see, for example, Patent Literature 1).
The heat pump apparatus includes the primary-side refrigeration cycle including a compressor, a heat source-side heat exchanger, an expansion valve, a four-way valve, and a cascade heat exchanger, and the secondary-side refrigeration cycle including a load-side heat exchanger, a pump configured to send water, a four-way valve, and the cascade heat exchanger. The cascade heat exchanger includes fluid passages each including a plurality of perforated flat tubes and small-diameter heat transfer tubes in combination, and a hydraulic diameter of each of the fluid passages is 3 mm or less and 0.5 mm or more.